There are well-known methods for recombustion of the fumes discharged so as to reduce the NOx emissions, such as those described in patents U.S. Pat. No. 5,139,755 and WO97/25,134.
This recombustion is a nitrogen oxides reduction technique based on stepping of the combustion. A furnace usually contained in these boilers, wherein this technique is used, comprises three zones:
the first zone wherein about 85 to 95% by mass of the fuel is burnt under standard conditions, i.e. with about 5 to 15% excess air when gaseous fuels or liquid fuels are used;
in the second zone, downstream from the first zone, the remaining fuel which consumes the excess oxygen of the fumes from the first zone is injected. The atmosphere of this second zone becomes reducing and the nitrogen oxides generated in the first zone are essentially converted to molecular nitrogen under the action of hydrocarbon-containing radicals;
in a third zone (or postcombustion zone), air is added so as to eliminate all the unburnt substances generated in the second zone and to have a standard excess air of 5 to 15% at the outlet.
The recombustion method described above generally allows to decrease the NOx emissions by about 50 to 80%.
Although in its principle recombustion is attractive as regards performances and insofar as it requires no reactant allowing to reduce the nitrogen oxides other than the fuel itself, it however presents considerable drawbacks.
In the case of a thermal generator working with natural gas, the most significant difficulties encountered are:
high corrosion in the recombustion zone and in the postcombustion zone, due to the presence of a reducing atmosphere or to alternating oxidizing and reducing atmospheres,
imperfect oxidation of the recombustion fuel in the postcombustion zone and, as a consequence, formation of gaseous and solid unburnt residues and, in the most severe cases, fouling of the downstream exchange surfaces, which reduces the overall energy efficiency and requires more sophisticated and expensive automatic cleaning equipments,
safety difficult to provide because of the stage of injection of a fuel in the recombustion zone. The main risk is a non-combustion of said fuel owing to an operating trouble or to an ill-controlled transient operation. In this case, there are risks of explosion in the parts situated downstream from the recombustion zone (recovery boiler, filter, etc.).
In the case of a thermal generator working with heavy petroleum products, the implementation difficulties are the same as those encountered with natural gas, but they are often increased for some of them. This is in particular the case for solid unburnt residues produced in larger quantities. Furthermore, the denitrification efficiency can be lower with these heavy products because of the presence of nitrogen-containing compounds in the initial fuel.